The present invention relates to large area semiconductor devices and to flat arrays of semiconductor devices such as solar cells and to methods of manufacturing such devices and arrays from silicon.
In general, semiconductor devices such as rectifiers, solar cells, transistors and the like are formed by junctions between semiconductor layers of different conductivity types. Such devices when made of silicon are generally manufactured by starting with a monocrystalline wafer of silicon sliced from a rod which was doped while it was being grown to make it either n-type or p-type. One face of the wafer or a portion thereof is then converted to the opposite conductivity type resulting in a p-n junction. The conversion from one conductivity type to the other in the wafer is accomplished by introducing a dopant element of the desired type either by diffusion, ion implantation or by growing a doped epitaxial layer on the wafer surface. Electrical interconnections, insulation and additional junctions are added by a variety of techniques well known to those skilled in the art.
In the production of large area arrays such as power rectifiers or solar cells, several disadvantages are inherent in prior art techniques. Particularly for solar cells, where extremely large areas are desired for converting solar energy to electrical energy, the cost of such devices is prohibitive unless no other energy source is available. First, a high purity polycrystalline silicon rod must be grown. The rod must then be converted to monocrystalline material by float zoning or the Czochralski process. The rod must then be sawed into wafers which must be lapped, cleaned and polished. Waste occurs in all of these steps before formation of an active semiconductor device having p-n junctions can begin. Even then the single array size is limited by diameter of the semiconductor rod that is sliced to form the wafer.
For some time, therefore, thought has been given to developing cheaper methods of producing solar cells. One approach which has been tried but found to result in an extremely inefficient unit is the use of bulk polycrystalline material. Grain boundaries between crystallites in such materials prevent proper transfer of charges in the material. Dopants tend to follow grain boundaries when diffused into the material. Accordingly unoriented p-n junctions appear around grains effectively isolating charges. Further, heavy metal impurities tend to concentrate in the grain boundaries and along with discontinuities in the grain boundaries contribute to recombining of electron-hole pairs resulting in no electrical output from the system. Accordingly, this approach has not been accepted.